Trial pits are formed for purposes of exploration for petroleum, geothermal energy, minerals, etc. Since these pits are as deep as 500 to 5000 m, lost circulation, or lost returns, may take place during a boring operation. If this phenomenon occurs, the drilling mud escapes into the earth through porous sidewalls, making it impossible to retain the head of the drilling mud. This may bring the porous sidewalls to destruction.
In order to plug up the gap that causes such lost circulation, chemicals are supplied to the location of the lost circulation. Originally, a supply pipe was inserted into the ground to supply the chemicals. Specifically, the chemicals are conveyed through the pipe in such a way that drilling mud is followed by the chemicals, thus forming so-called mixed-phase fluid. However, chemicals for stopping lost circulation differ from drilling mud in specific gravity, viscosity, surface tension, and other characteristics. Therefore, when chemicals which are not diffused are employed, they move downward through drilling mud. During this process, the chemicals are not mixed with the drilling mud, nor is a mass of fluid formed. Wrinkles are formed inside the pipe at the rear end. The chemicals move down the pipe while causing the wrinkles to vibrate. Therefore, a small mass is torn out of the mass of fluid at the position of one rear wrinkle. This small mass drifts within the pipe, increasing the area with which the chemicals come into contact with the drilling mud. This is undesirable for the chemicals that should be conveyed without being mixed with drilling mud. Thus, this supply is a wasteful method, and in which it is difficult to pump an almost complete mass of fluid through the pipe to the location of lost circulation.
In an attempt to avoid this problem, i.e., to convey chemicals in the form of a mass to a desired place without reducing the amount of the chemicals, capsules charged with chemicals have been used. These capsules permit chemicals to be transported without being mixed with drilling mud. One method heretofore proposed to diffuse chemicals out of the capsule that has reached the bottom of a hole is to destroy the whole capsule. Another proposed method is to open the valve mounted at the front end of the capsule.
When the former method is adopted, a destruct mechanism such as an explosive is needed, which requires careful handling. Further, porous sidewalls may be destroyed, depending on the destruct mechanism. When the latter method is utilized, it is not assured that the valve at the front end of the capsule is opened with certainty, because of the water pressure inside the hole, the natures of the drilling mud and slime, and other factors.
In addition, boreholes are relatively rarely vertical. Some boreholes are inclined at 30.degree. or 45.degree. . Also, porous sidewalls are not flat but rough. Accordingly, in order to allow the capsule to drop smoothly, the capsule must have a self-guiding function. At depths of hundreds to thousands of meters, a gap is produced between members of different kinds of the capsule because they are caused to expand or contract by high temperature and high pressure. As a result, after the capsule is used only once, a distortion may be produced, or the contact portions of members may be damaged. This makes it impossible to repeatedly use the capsule.